In 2006 the IAU (international Astronomical Union) held a meeting, IAU0603, and resolution 5A discussed the “Definition of a Planet”.

Courtesy the IAU.

RESOLUTIONS
Resolution 5A is the principal definition for the IAU usage of “planet” and related terms.
Resolution 6A creates for IAU usage a new class of objects, for which Pluto is the prototype. The IAU will set up a process to name these objects.
IAU Resolution: Definition of a “Planet” in the Solar System
Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation “planets”. The word “planet” originally described “wanderers” that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information.
RESOLUTION 5A
The IAU therefore resolves that planets and other bodies in our Solar System, except satellites, be defined into three distinct categories in the following way:
(1) A “planet”1 is a celestial body that
(a) is in orbit around the Sun,
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
(c) has cleared the neighbourhood around its orbit.
(2) A “dwarf planet” is a celestial body that
(a) is in orbit around the Sun,
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape2,
(c) has not cleared the neighbourhood around its orbit, and
(d) is not a satellite.
(3) All other objects, except satellites, orbiting the Sun shall be referred to collectively as “Small Solar-System Bodies”.

So, there you have it, by the above definitions Pluto was no longer a planet, but a dwarf planet as dictated by Resolution 6A

IAU Resolution: Pluto
RESOLUTION 6A
The IAU further resolves:
Pluto is a “dwarf planet” by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects.

But does this really resolve the issue of what a planet actually is? Clearly, the answer is NO.

Neil De Grasse-Tyson may wear the T-shirt “Pluto Killer”, and the veritable Mike Brown often gets referred to as the Pluto Killer, but in Mike’s case this is unfair, he simply continues to find bodies that are in the scattered disk, the Kuiper belt and possibly Oort cloud that confound and confuse due to their sizes, masses and general nature, in Neil’s case, his ego likes to use the epithet of Pluto Killer at every opportunity and in my humble opinion, he is just WRONG.

The IAU have not solved the issue, by their definition, no object orbiting any other star is a planet, because R5A(2a) requires it to be orbiting about the Sun, it is clear to even the most absent minded that an exoplanet does not orbit the Sun, it orbits another star – are these objects no less planets?

Further, when I was a child of 10, some 47 years ago, I was in constant communication with the late and amazing Sir Patrick Moore, I suggested to him that there may be planets orbiting the galaxy between the stars, perhaps some formed there, perhaps some were kicked out of planetary systems by planetary collisions or stellar collisions – Patrick, in his inimitable way, politely poured cold water on this notion and the notion was duly put to bed – but we now know that planets do exist between the stars, so called Rogue Planets, some have been detected by stellar occultation and some by micro-gravitational lensing – however, these bodies do not orbit the Sun, they are not stellar mass objects, not even brown dwarves, so what are they? If they are the mass and size of Earth, or larger, then they can only be called a planet, because they are most certainly not comets, asteroids, stars or dark matter.

Now let’s come nearer to home, Mike Brown and Konstantin Batygin, through diligent and brilliant research, have laid the groundwork for the existence of a planet in the very outer reaches of the solar system, Planet 9, which may be up to 250 times further from the Sun than Earth, and could have a mass up to 17 times that of Earth. The idea was first posited by Chad Trujillo and Scott Shepherd, and they have recently published some updated research on this very topic. A synopsis of this can be found here.

Whilst this planet may meet all the criteria set out by the IAU for the definition of a planet, it is highly likely that other bodies may be out there which are not as massive, or maybe even as distance, to this end, research by Kat Volk and colleagues at the LPL (Arizona University Lunar and Planetary Lab) have proposed that a planet, between the mass of Mars and Earth, more likely nearer Mars, is orbiting in the outer fringes of the solar system, perhaps toward the inner edge of the Kuiper belt, and certainly closer than 100 times the Earth-Sun distance (100AU).

A Mars sized object in this region of the solar system would have had insufficient time to clear its orbit, as required by the IAU definition of planet R5(1c), and thus would be a dwarf planet under this definition – a clearly ridiculous situation as it would then beg the question as to whether Mars should remain defined as a planet – clearly nonsense.

What is interesting, if there is a planet the same mass as Earth around 100+AU from the Sun, then it too would have had insufficient time to clear its orbit as required by R5(1c), and thus we would have a situation where an Earth mass planet is not a planet!!

Our old definition of planet was always open for discussion, all terms should be open for adaption to suit new observations and information, and they should also do their level best to avoid discord and confusion – I think we can all agree that the current definition causes both discord and confusion.

What we need is a new IAU resolution for the definitions of planets – it is clear the array of planets we shall find will necessitate the sub-classification of basic planet types just as we do for stars – once thought to be a simple process, as time moved on this has become increasingly complex, but the basic stellar classifications have remained.

IAU Resolution XXXXXX: Definition of a Planet

  1. A planet is an object that meets the following criteria.
    1. Has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium shape, nearly round, and has a diameter not less than 644km (400 miles)
    2. Orbits a star in a cleared or semi-cleared orbit.
    3. May orbit the galaxy outside the confines of a planetary system.
  2. The term planet is further sub-divided as follows.
    1. Ice dwarf – any body that exceeds 644km diameter but is composed primarily of ices, silicates and metals may not exceed 20% composition
    2. Dwarf – any body that exceeds 644km diameter, whose composition exceeds 20% metals or silicates.
    3. Terrestrial
      1. Has no atmosphere
      2. Has an atmosphere not exceeding 0.5% of the mass of the body
      3. Has a frozen surface (The composition of ices on the surface are not important)
      4. Has a liquid surface (The composition of liquids on the surface are not important)
    4. Ice Giant (4-20 Earth Masses – exact boundaries to be a moving target as knowledge increases)
    5. Gas dwarf (0.5 Saturn Mass to 4 times Jovian mass)
    6. Gas Giant (4.1x Jovian Mass to the lower brown Dwarf limit)

Further, the type of planet should have a suffix as follows, where this is appropriate.

      1. H – Hot, where the planet is orbiting so close to its star its surface temperature exceeds 800°C.

      2. Gw – Galactic wanderer, where the planet orbits no known star

      3. R – ringed, where the planet has been confirmed to possess a ring system

      4. U – the planet has an unusual orbit – such as those that orbit at extreme inclinations to the plane of the system

Obviously, this may not be the best way to define planets, it should be open to a full discussion and a resolution that satisfies everyone needs, but the exist definition is simply ridiculous and unworkable.

It has been mooted that one reason for the change by the IAU was that people did not want to confuse children by adding Ceres, Eris, Makemake and Haumea planets, if this was a reason, it is pretty unscientific and rather childish to be honest. At the moment, there are possibly as many as 16 objects known, which the IAU define as dwarf planets, but by a much more thought through definition should be classed as planets, albeit with sub classifications which would make several Ice Dwarves and others just Dwarves.

On a personal note, I have always considered Pluto a planet, regardless of the IAU, just because they say it is a dwarf planet, does not diminish its planet status, the Sun is a dwarf star, it is no less a star because we say it is a dwarf – any intelligent life living on a planet orbiting a Red Dwarf may consider our star a giant, even though we do not, perspective is king.

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